δ-Opioid receptor antagonism induces NMDA receptor-dependent excitotoxicity in anoxic turtle cortex

Abstract

SUMMARY δ-Opioid receptor (DOR) activation is neuroprotective against short-term anoxic insults in the mammalian brain. This protection may be conferred by inhibition of N-methyl-d-aspartate receptors(NMDARs), whose over-activation during anoxia otherwise leads to a deleterious accumulation of cytosolic calcium ([Ca2+]c), severe membrane potential (Em) depolarization and excitotoxic cell death (ECD). Conversely, NMDAR activity is decreased by ∼50% with anoxia in the cortex of the painted turtle, and large elevations in[Ca2+]c, severe Em depolarization and ECD are avoided. DORs are expressed in high quantity throughout the turtle brain relative to the mammalian brain; however, the role of DORs in anoxic NMDAR regulation has not been investigated in turtles. We examined the effect of DOR blockade with naltrindole (1–10 μmol l–1) on Em, NMDAR activity and [Ca2+]chomeostasis in turtle cortical neurons during normoxia and the transition to anoxia. Naltrindole potentiated normoxic NMDAR currents by 78±5% and increased [Ca2+]c by 13±4%. Anoxic neurons treated with naltrindole were strongly depolarized, NMDAR currents were potentiated by 70±15%, and [Ca2+]c increased 5-fold compared with anoxic controls. Following naltrindole washout, Em remained depolarized and [Ca2+]cbecame further elevated in all neurons. The naltrindole-mediated depolarization and increased [Ca2+]c were prevented by NMDAR antagonism or by perfusion of the Gi protein agonist mastoparan-7, which also reversed the naltrindole-mediated potentiation of NMDAR currents. Together, these data suggest that DORs mediate NMDAR activity in a Gi-dependent manner and prevent deleterious NMDAR-mediated[Ca2+]c influx during anoxic insults in the turtle cortex.